2) Place a shelf in the centre of the oven, and preheat the oven to 180C/Gas 4. Butter 2 round cake tins, line the bottoms with circles of greaseproof paper and butter the paper. Dust with flour, shaking off the excess. Set aside.
3) In the bowl of a standing mixer with the paddle attachment, add the cold butter, sugar, eggs and vanilla. Beat on slow to combine and then raise to medium-high speed for 8 to 10 minutes until the butter is almost completely incorporated into the mixture.
4) Meanwhile, in a medium bowl, sift the flour, bicarbonate of soda, baking powder and salt. With the mixer on low speed alternately add the flour mixture and the cocoa liquid in 2 additions of each, stopping to scrape the bowl occasionally and mixing just until combined.
5) Evenly divide the batter between the 2 cake tins (you can weigh them to get them even). Bake for 45 to 55 minutes, until the cake springs back when gently pressed and a cocktail stick inserted in the centre comes out clean. Cool in the tins for 10 minutes, then place on wire racks to finish cooling completely.
For the icing:
1) Put the chocolate chips in a heatproof bowl and place over a saucepan of simmering water; do not let the water touch the bottom of the bowl. Stir the chips occasionally until melted, then remove from the heat to cool.
2) In a medium mixing bowl, sift together the icing sugar, cocoa and salt and set aside.
3) In the bowl of a standing mixer with the paddle attachment, beat the butter and the cream cheese until light and fluffy. On low speed, slowly add the icing sugar mixture to the butter mixture, combining well.
4) Add the melted chocolate and the sour cream and vanilla, mixing just until combined. It's best to use the icing as soon as soon as it is made.
5) If the cakes are a little uneven, level the top of each. Using a serrated knife, split the first cake into two layers and place one on a cake plate. Spread about 1/4 of the icing on top of the layer and smooth to the edges.
6) Add the next layer and spread icing over it. Split the other cake into layers, add the bottom half on top of the icing, and spread with another 1/4 of the icing as before.
7) Turn the remaining layer of cake upside down, so that the bottom is on the top, and add it to the top of the layers. Pile on the rest of the icing, evenly spreading it all over the top and the sides of the cake, swirling as you go. Yum!
Cook's Notes: I place a 30cm wooden skewer in the centre of the cake to secure it for layering, removing it before icing the cake. To keep the cake plate clean while you are icing, lay strips of waxed paper under the cake.